The invention is directed to sensors which employ either the anisotropic magneto-resistive effect (AMR) or the gigantic magneto-resistive effect (GMR) and are suitable for indicating magnetic fields. In particular, the sensor elements improved by the invention are employed for automatically identifying when a characteristic value is exceeded. For example, a particular field strength can be exceeded if a permanent magnet approaches in the position of the sensor, or when an increasing electric current flows through an electric conductor close to a sensor. In both cases, a switching operation can be initiated by the sensor when the characteristic value of its output signal is exceeded. Information about the position of the magnet or the current can be most easily obtained in the vicinity of the switching point, if a linear correlation with the sensor signal exists.
Sensors of the aforedescribed type are known in the art, however, their linearity and temperature independence are not adequate for the applications. For obviating these deficiencies in the case of AMR-sensors, EP 0 796 491 describes a circuit capable of compensating the non-linearity of the characteristic curve. However, the described circuit is complex and too expensive for many applications.
The changes in the resistance value of an AMR strip or a GMR strip in a magnetic field are typically nonlinear. For an AMR strip, where the magnetic fields is applied in the plane of the strip perpendicular to the current direction, the resistance changes with the square of the field strength. For a GMR strip, the resistance also changes with the square of the field strength for a spin-valve layer system, if the magnetization of the pinned ferromagnetic layer coincides with the longitudinal direction of the strips and the magnetic field is oriented perpendicular thereto. Sensors constructed of such strip resistors and arranged as Wheatstone bridges provide output signals that have essentially the same quadratic dependence on the magnetic field as the resistors.
Several patents already describe approaches to improve the linearity of sensor elements implemented as AMR sensors. For example, the published application DE 26 15 539 proposes to cover the magneto-resistive layer strips of the sensor resistors with a plurality of closely spaced non-magnetic layer strips with a high conductivity, wherein the longitudinal direction of the non-magnetic layer strips forms an angle of approximately 45° with the longitudinal direction of the magneto-resistive layer strips. This produces a linear characteristic curve for small positive and negative magnetic fields. The slope of the conductive layer strips is opposite to the slope of the magneto-resistive layer strips, which can also cause the resistance to change in opposite directions in the same magnetic field. The economic patent DD 260 773 proposes to expand the field range of the linear characteristic curve by constructing each magneto-resistive resistor from two parts having opposite slope of the high-conductivity layer strips while the angle of the remanence of the magneto-resistive strips can rotate differently. The different rotation can be achieved by different strip widths. In sensors where the characteristic curve is linearized by inclining the conductive layer strips on the magneto-resistive strips, the conductive layer strips disadvantageously lower the resistance values of the magneto-resistive strips to approximately one-third. This necessitates an increase of the sensor area by approximately the same factor, which is not economical. In addition, conductive layer strips with smaller widths in the range of approximately 1 μm and tolerances of no more than a few percent must be produced with a high accuracy in the slope angle. This is technologically very challenging. The aforementioned method for linearization can only be applied to AMR sensors because it is based on a change in the current direction in the resistive strips. This principle does not affect the resistance value of GMR sensors.
The temperature dependence of the output signal of the magneto-resistive sensors can be eliminated according to the document GB 2 281 654 by applying a thermometer layer on the substrate directly underneath the magneto-resistive layer and by correcting the sensor signal with an electronic circuit. According to the patent JP 63 179 586, a magneto-resistive layer with the same parameters as the field sensor arrangement is used as the thermometer layer. Both approaches disadvantageously require complex electronic circuitry and are difficult to adjust.
U.S. Pat. No. 4,506,220 describes an attempt to eliminate the temperature dependence of the sensor signal by arranging in each arm of the sensor bridge employing the same magneto-resistive material one resistor that is dependent of the magnetic field and another resistor that is independent of the magnetic field. The resistors that depend on the magnetic field are arranged in direct opposition in the bridge circuit and are connected in one point. This attempt, however, only prevents a common-mode offset at a different temperature. The temperature coefficient of the resistance change in a magnetic field is not changed, so that the bridge signal still changes with temperature.
In an arrangement proposed in the patent application EP 0 048 289, conductive sublayers with a negative temperature coefficient of the resistance are disposed underneath all magneto-resistive layers of the sensor which have a positive temperature coefficients of the resistance. The temperature coefficient of the double layer can be adjusted to zero by selecting a suitable ratio of the layer thicknesses of the magneto-resistive layers and the sublayers. The bridge resistance of a magneto-resistive sensor configured in a Wheatstone bridge can then also be made independent of the temperature. However, it would be a mistake to conclude that the signal of the sensor corresponding to the applied magnetic field is also the same at all temperatures. The temperature coefficient of the change of the resistance in the magnetic field for magneto-resistive layers has a negative value, whereas the temperature coefficient of the resistance has a positive value. The output signal of the magneto-resistive sensor decreases with increasing temperature, unless compensated. The sublayers disclosed in EP 0 048 289 are connected electrically in parallel with the resistors of the magneto-resistive layers. The change in resistance of the double layers that determines the signal when a magnetic field is applied, is therefore smaller than with only a single magneto-resistive layer. With increasing temperature, the resistance of the magneto-resistive layer and the resistance of the sublayer decreases. As a result, the change in resistance caused by the magnetic field also decreases. In total, one obtains a sensor output signal which increases even more with increasing temperature than the signal of a sensor which only consists of magneto-resistive layers.